Numerous mechanisms have been proposed as contributing factors in the etiology of diabetic cardiomyopathy, ranging from neurohumoral imbalances and extracellular remodeling, to perturbations in the intrinsic properties of cardiomyocytes. In the latter case, imbalances in rates of damage (e.g., oxidative) and replacement (i.e., turnover) of cellular constituents (e.g., proteins, mitochondria) have been implicated in the development of cardiac dysfunction during diabetes. Although many studies have investigated the role of increased oxidative stress, little is known regarding how diabetes impairs the turnover of damaged cellular constituents. Turnover of cellular constituents exhibits a striking time-of-day-dependent variation, which is governed by the cardiomyocyte circadian clock. Moreover, genetic disruption of the clock in the heart temporally suspends these processes, leading to development of dilated cardiomyopathy. Compelling evidence presented within this application suggests that both autophagy and mitophagy (autophagy of mitochondria), processes critical in the repair/replacement of cellular constituents, are circadian regulated in the heart. Our investigation of the cardiomyocyte circadian clock further revealed that the posttranslational modification, protein O-GlcNAcylation, is integral to the clock mechanism; the importance of this relationship is highlighted during diabetes (both type 1 and 2), when chronic elevation of cardiac protein O-GlcNAcylation (secondary to aberrant glucose metabolism) is associated with a phase shift in the clock within the heart. We postulate therefore that disruption of the clock-O-GlcNAc relationship during diabetes causes temporal misalignment of cardiac processes involved in repair/replacement of cellular constituents. These studies have led to the hypothesis that chronic disruption of the clock-O-GlcNAc relationship in the heart during T2DM impairs temporal partitioning of autophagy/mitophagy, ultimately impairing cellular constituent quality control leading to contractile function. In order to test this hypothesis, three Specific Aims are proposed. Aim 1: Demonstrate that the cardiomyocyte circadian clock modulates quality control of cellular constituents through transcriptional and posttranslational regulation of autophagy/mitophagy mediators (Physiologic/Mechanistic Aim). Aim 2: Demonstrate that chronic disruption of the clock-O- GlcNAc relationship during T2DM impairs quality control of cellular constituents through attenuated temporal partitioning of autophagy/mitophagy (Pathologic Aim). Aim 3: Demonstrate that behavior- and/or pharmacologic- mediated normalization of the clock-O-GlcNAc relationship during T2DM attenuates development of cardiac dysfunction (Therapeutic Aim). Successful completion of the proposed studies will lead to new fundamental insights regarding the causal role of circadian disruption in the etiology of diabetic cardiomyopathy, and will help identify innovative approaches for reducing the risk of cardiac dysfunction in diabetic patients.